High-Level N Management

It's been said many times that a picture is worth a thousand words. But the right picture may actually be worth something that corn growers can take to the bank.

For the past six years, Alfred Blackmer and a team of Iowa State University researchers have been looking at aerial photos of cornfields. Their goal: to study nitrogen distribution patterns and to discern whether corn is suffering from nitrogen deficiency. Iowa's Corn and Soybean Promotion Boards have helped fund the study.

"For years we've watched corn plants from the field edge," he says. "Corn with sufficient nitrogen is a deep green color. If the crop is severely nitrogen-deficient and we look carefully, we can see the yellow leaf symptoms in the lower leaves.

"But we've found that if the deficiency is only slight, the entire plant may just be a lighter shade of green. If the entire field is deficient, it's difficult to see. And comparing the color of plants from field to field doesn't help, since different hybrids are often different shades of green."

Some crop consultants use chlorophyll meters to look for nitrogen deficiencies, since the amount of chlorophyll (and thus green color) increases with available nitrogen. But chlorophyll meters look at just one leaf at a time. With aerial remote sensing, you see a photo of the entire field, shot at about 2,000' above the field.

"You need to be just high enough that you can see the whole field," Blackmer says. "Then you can look for differences in the intensity of the color of the field. You can't see individual plants from this altitude, but nitrogen deficiencies and poor nitrogen distribution show up easily."

Blackmer suggests that growers try remote sensing if they're concerned about nitrogen losses or are trying to maximize yields with a minimum amount of applied nitrogen.

"You don't need expensive equipment to do this," he says. "All you need is a reasonably good camera, color print film and a way to get above the field to shoot photos."

If you think you might try remote sensing, Blackmer has a suggestion: Make reference strips by applying nitrogen at lower and higher rates than you're using on the rest of the field. Use strips at least six rows wide, since the wider the strip, the easier it is to pick out differences.

"Once you get your field photos, you can probably locate the low rate by its lighter color," he says.

He points out that excess nitrogen doesn't show up in remote sensing. So if you can't see a difference in color between the strip where you increased the rate and the rest of the field, the rate in the rest of the field probably was adequate. If you can't locate the lighter-colored strip where you underapplied nitrogen, there's a chance that the lower rate was sufficient, says Blackmer.